Open recirculating cooling water systems are widely used processes for rejection of waste heat from a variety of industrial processes. Such systems are open as water e.g. evaporates at the cooling tower. In addition, controlled removal of recirculating water is necessary to limit the accumulation of dissolved species that cause corrosion, scaling and fouling. The effluent water is removed with the so-called “blowdown stream”.
Various additives are on the market that can be added to the recirculating water in order to specifically avoid corrosion, scaling or fouling. These additives are normally fed at a feeding rate needed to maintain a relatively constant concentration in the recirculating water. The feeding rate is typically controlled to replace the amount of the additives that are consumed within the recirculating system and that are removed with the blowdown stream.
However, this type of deposit control is static and cannot properly react when the composition of the recirculating water changes unexpectedly. Such unexpected changes can have various causes. For example, the temperature and thus also the composition of the fresh water (makeup water) that is added to the system varies over the year.
Various attempts have been made to monitor the properties of the recirculating water in order to better understand the processes taking place and to properly react in case of unexpected events. All these attempts rely on the monitoring of one or more key operation indicators such as pH value of the recirculating water, electrical conductivity of the recirculating water, concentration of anti-deposit additives in the recirculating water.
Based upon the measured values of these key operation indicators, conclusions are drawn in respect of the current tendency of the recirculating water to form deposits. If necessary, counter measures are initiated that are believed to be appropriate, such as increasing the dosage of a particular anti-deposit additive. Attempts have also been made to simultaneously monitor a plurality of key operation indicators in order to base the initiated counter measure on a broader basis of data.
However, key operation indicators such as pH value, electrical conductivity, and the like are not directly linked to deposit formation. Even if electrical conductivity and pH value are stable over time, undesired scaling may occur. Ongoing processes may compensate one another. For example, when the pH value is decreased for some reason, this may lead to an increase of the concentration of e.g. basic CaCO3 in the recirculating water thus increasing the pH value again. Furthermore, a sudden change of the pH value, for example, can have various reasons. The pump that supplies acid or base to the recirculating water may be broken, the pH meter may be broken, the storage tank containing acid, base or buffer may be empty, and the like. Therefore, a key operation indicator may change for various reasons that all have the same consequence of undesired deposit formation.
US 2009/0277841 discloses a process for operation of evaporative recirculating cooling systems. In addition to reducing the scaling and corrosive tendencies of the water, the method is said to eliminate or reduce discharge from the system without creating any localized corrosive or scaling conditions as a result of the treatment process. The described measurement and control system generally comprises an array of measurements, a means of implementing control logic, and an array of control actions including activating an ion exchange device to treat makeup water. Preferably, the measurements include one or more of pH, conductivity, hardness, alkalinity, corrosiveness, scaling tendency, treatment additive dosage level, and treatment additive residual of the makeup, treated makeup, and recirculating water.
US 2010/0176060 and US 2013/0026105 disclose the control of scaling in a cooling water system with CO2 based upon measurements of the cooling water's pH, alkalinity and Ca2+ concentration.
A further problem is that the formation of certain deposits is irreversible. This is particularly the case for scaling. While sophisticated anti-scaling additives are available on the market that are capable of effectively avoiding the deposition of the scaling on surfaces at appropriate dosages, they are usually not capable of removing the scaling once it has been deposited. In consequence, the dosage of anti-scaling additives in the recirculating water is typically kept higher than really necessary in order to avoid scaling formation, just to ensure that no scaling is irreversibly formed.